I'm trying to understand if we can lock the Input and Edit screens for Deal, Lead and Quote. I've re-arranged the way I want each screen to appear to the users and don't want them to change this. Is this possible? How?

A second question is with fields "Commencement Date" and "Assumed Close Date" can't make them visible during the creation or edit of a Deal ... but they appear when viewing the deal on the top section. Can they be made visible for input when creating a new deal or we edit an exisitng one?

You can use "save preferences for all users" - by default your users (sales agents) will get the form view you have just configured, but they can always reset settings - in this case they will get the default form. At this moment there is no option to lock the configured form for other users, only save.